Pulse oximeters are well known devices used to measure the oxygen saturation of arterial blood where oxygen saturation is defined as the ratio of the total oxygen carrying capacity of arterial blood, to its actual oxygen content when measured. Known pulse oximeters calculate oxygen saturation by measuring the ratio of oxygenated hemoglobin molecules in the blood to the total number of hemoglobin molecules which are present in the same blood sample.
It is well-known that non-oxygenated hemoglobin molecules absorb more red light than oxygenated hemoglobin, and that absorption of infrared light is not affected by the presence of oxygen in the hemoglobin molecules. For this reason, all known pulse oximeters commence the oxygen saturation measurement process by directing both a visible red (VR) and an infrared (IR) light source through a blood carrying tissue sample, and thereafter known pulse oximeters detect and process the signal received after passage through the tissue sample.
More particularly, one of the earlier known techniques to measure oxygen saturation is described in a Japanese patent application to Takuo Aoyagi, which application was laid upon for publication on Oct. 9, 1973, in Laid Open Japanese Patent Publication Number SHO 50/1975-128387. Aoyagi describes an oximeter which includes an incandescent broad band light source and two separate photo detection circuits, one photo detector being overlayed with an optical filter such that it is only sensitive in the red frequency range, and the second photo detector being overlayed with a second optical filter, such that it is only sensitive in the infrared frequency range. The light energy from the light source is transmitted through a blood bearing specimen such as a finger or ear lobe, and the amount of light transmitted through the specimen is detected by two separate photo detection circuits.
Each signal from each detector circuit is separable into a constant component and a pulsatile component. The constant component, or DC component, is indicative of non-pulsating blood flow in the specimen area, while the pulsatile component, or AC component, is indicative of the pulsating blood flow in the specimen area.
A first and second calculation circuit, operating on both the red and infrared signal branches, divides the AC (pulsatile) portion of the signal, by the DC (steady state) portion of the signal, in order to standardize the amplitude of the AC portion. Thereafter, a third calculation is performed wherein the standardized AC portion of the red signal is divided by the standardized portion of the infrared signal and this ratio is indicative of the oxygen saturation level.
Although Aoyagi was one of the earlier known techniques to measure the oxygen saturation level in blood, the necessity to perform three separate calculations, and perform those calculations with analog, rather than digital circuitry, resulted in a relatively inaccurate measuring technique that also had a slow response time.
A second device for measuring the oxygen saturation level of the blood is set forth in U.S. Pat. No. 4,407,290 to Wilber, issued on Oct. 4, 1983. Wilber describes a blood constituent measuring device capable of determining the concentration of certain blood constituents such as hemoglobin and oxyhemoglobin, and use of that information to determine oxygen saturation of the blood. More particularly, an AC modulated pulse train is developed, which pulse train is indicative of light transmitted through a tissue sample at both a red and infrared wavelength. Wilber specifically teaches that the received pulses are normalized by scaling both the AC and DC components of each light source signal, so that the DC (average) component from each light source is made equal to a known preset level. This normalization procedure is required in Wilber, in order to accomplish an accurate subtraction of the DC component of each signal in each channel from the total signal, so that the resultant signal is essentially only an AC component on a zero reference level. After the DC components are removed, the AC signal components are multiplexed and converted to digital form for processing in a digital processor.
Although the Wilber patent has certain advantages over the prior techniques of Aoyagi, such as digital processing, it still suffers from the disadvantage that it must normalize the received signals in order to obtain a relatively accurate measurement of oxygen saturation levels. The necessity to normalize such signals inherently delays the measurement process resulting in a relatively slow response time for the device, and also substantially increases cost due to the extra circuitry required.
A final, and more advanced, technique in the area of pulse oximeters is described in U.S. Pat. No. 4,759,369, which issued to Andrew C. Taylor on Jul. 26, 1988, which patent is assigned to the same assignee as the instant invention. The teachings of U.S. Pat. No. 4,759,369 are specifically incorporated herein by reference. The Taylor device attempts to overcome the disadvantages, such as those described in the Aoyagi and Wilber references, by providing a simplified pulse oximeter design with improved accuracy and reduced calculation time. More particularly, the Taylor patent teaches the use of first and second light sources, the first light source generating energy in a red wavelength and the second light source generating energy in an infrared wavelength. The light sources are directed through a blood carrying tissue sample and the amount of light transmitted through the tissue sample is detected by a photo detector. The signals (red and infrared) received by the photo detector, are separated into a constant (DC) component and a pulsatile, i.e. time varying (AC) component. The AC portion of the signal is divided by the total signal (AC and DC) to standardize the AC signal, and the standardized AC red signal is then divided by the standardized AC infrared signal to obtain the necessary ratio for the determination of oxygen saturation.
A specific object and feature of the Taylor invention was the necessity to scale the level of the input signals in order to insure that those input signals did not exceed the dynamic range of an analog/digital (A/D) converter included within the circuit, which analog/dialog converter functioned to convert the AC analog signals into a digital signal suitable for processing by an attendant digital computer. This scaling feature is accomplished by two separate gain control circuits which function to adjust the drive currents to the red and infrared light sources. The adjustment process effects the entire signal (AC and DC), to ensure that this signal falls within a range which can be accommodated by the A/D converter.
Although the Taylor device is superior to other prior art pulse oximeter devices, the necessity to scale input signals so that those signals can be accommodated by the range of an A/D converter reduces both the inherent accuracy and response time of the device described in the Taylor reference.
In addition to the foregoing limitations described with respect to the Aoyagi, Wilber and Taylor devices, such prior art pulse oximeters have several additional disadvantages. First, all three devices utilize the hardware portion of the circuitry to separate the DC component from the AC component for both the IR and VR channels, with the AC component being amplified separately from the DC component. As each separate amplifier does not provide exactly the same amount of gain, inaccuracies are introduced into the measurement process when the ratio of the AC component to the DC component is calculated. This "channel matching" problem then results in an inherent inaccuracy in the calculated oxygen saturation level.
Also all three prior art devices do not effectively discriminate against motion artifacts (or other aberrant input data), where a motion artifact is defined as a large false signal generated when there is inadvertent relative movement between the tissue sample and the input sensors. Failure to detect and effectively eliminate aberrant input data in such devices, can result in inaccurate information being provided to an operator of the pulse oximeter.
It is, therefore, an object of the instant invention to provide a pulse oximeter device capable of greater accuracy then the prior art device described above, along with the ability to provide an improved response time for processing incoming information and displaying an oxygen saturation level.
It is a further object of the instant invention to eliminate inaccuracies resulting from channel matching errors when calculating an oxygen saturation level.
It is a still further object of the instant invention to provide means to effectively discriminate against motion artifacts and other aberrant input data in a pulse oximeter circuit.